The present invention relates to a cathode ray tube, and more particularly to a cathode ray tube provided with an electron gun assembly that suppresses resolution deterioration on the periphery of a screen.
An in-line color cathode ray tube of self-convergence type is provided with an in-line electron gun assembly for emitting three in-line electron beams traveling in the same horizontal plane. One of the beams is a center beam, and the others are side beams traveling along the center beam. The cathode ray tube is also provided with a deflection yoke for generating a non-uniform magnetic field, with which the electron beams emitted from the electron gun assembly are deflected. The three electron beams emitted from the electron gun assembly are converged by the main lens portion incorporated in the electron gun assembly and then self-converge on the entire screen by the action of a non-uniform magnetic field. This magnetic field is made by a pincushion-type horizontal deflecting magnetic field and a barrel-type vertical magnetic field.
Electron beams 6 passing through this non-uniform magnetic field undergo astigmatism. As shown in FIG. 1A, each electron beam is applied with the forces acting in the directions of arrows 11H and 11V by the pincushion-type magnetic field 10. When the electron beam 6 falls on the periphery of the phosphor screen, it forms a distorted beam spot 12 on the phosphor screen, as shown in FIG. 1B. This distortion is due to the deflection aberration that causes the electron beam 6 to excessively focus in the vertical direction, i.e., in the V-axis direction.
Hence, the beam spot 12 includes a halo portion 13A extended in the vertical direction and a core portion 13B extended in the horizontal direction, i.e., in the H-axis direction. This deflection aberration becomes significantly marked in accordance with an increase in the size of the tube or the deflection angle thereof, and results in a marked deterioration of the resolution on the periphery of the phosphor screen.
In order to provide a solution to the resolution deterioration that is due to the deflection aberration, a high-performance electron gun assembly has been developed. This electron gun assembly corrects the deflection aberration on the periphery of the screen by varying the lens power of an electron lens inside the electron gun assembly in accordance with the amount of deflection of an electron beam directed toward the screen periphery.
An example of such an electron gun assembly is described in Jpn. Pat. Appln. KOKAI Publication No. 64-38947. This electron gun assembly comprises a first grid G1, a second grid G2, a third grid G3, a fourth grid G4, a fifth grid G5, a first intermediate electrode GM1, a second intermediate electrode GM2, and a sixth grid G6. These elements are arranged from the side of a cathode K(R,G,B) to the side of a phosphor screen in the order mentioned, as shown in FIG. 2. The third to sixth grids are applied with voltages shown in FIG. 3.
Referring to FIG. 3, the solid line in the graph represents the voltage that is used in the non-deflection mode, i.e., in the mode wherein an electron beam is focused on the center of the phosphor screen. The broken line in the graph represents the voltage that is used in the deflection mode, i.e., in the mode wherein an electron beam is focused on the periphery of the phosphor screen. The axis of abscissa Z represents the positions of the electrodes arranged on the tube axis, which is substantially the central axis of a cylindrical neck portion in which the electron gun assembly is arranged. The increasing direction of the Z axis is a direction approaching the phosphor screen, while the decreasing direction is a direction approaching the cathode. The axis of ordinate V represents the voltage levels applied to the grids.
As shown in FIG. 3, the third and fifth grids are applied with a dynamic focusing voltage obtained by superimposing a variation voltage on a predetermined DC voltage Vf. The variation voltage varies in accordance with the amount of deflection of an electron beam.
When the voltage described above is applied to each grid, a quadrupole lens section QL2 is formed between the fifth grid G5 and the first intermediate electrode GM1, a cylindrical lens section CL between the fifth grid G5 and the sixth grid G6, and a quadrupole lens section QL1 between the second intermediate electrode GM2 and the sixth grid G6. The quadrupole lens section QL2 includes a vertical-direction component with a relatively focusing function and a horizontal-direction component with a relatively divergent function. The quadrupole lens section QL1 includes a vertical-direction component with a relatively divergent function and horizontal-direction component with a relatively focusing function. The main lens section ML of the electron gun assembly is constituted by the quadrupole lens sections QL1 and QL2 and the cylindrical lens section CL.
In the deflection mode, the voltages applied to the third and fifth grids are raised from the level indicated by the solid line to the level indicated by the broken line, as shown in FIG. 3. As shown in FIG. 4B, the power of the quadrupole lens section QL2 and that of the cylindrical lens section CL are suppressed. As a result, the diverging effect is maintained only in the vertical direction, with the focusing effect in the horizontal direction being unchanged. In this manner, an electron beam is prevented from being excessively focused in the vertical direction by the deflecting magnetic field.
However, the dynamic focusing voltage, which is synchronously related to the-horizontally-deflecting magnetic field, may fluctuate in synchronism with a deflecting frequency of 15 kHz or higher. When this fluctuation occurs, the capacitance between the fifth grid and the first intermediate electrode, that between the first and second intermediate electrodes and that between the second intermediate electrode and the sixth grid serve to conduct AC components. As a result, the first and second intermediate electrodes are applied with part of the dynamic focusing voltage acting in the horizontal direction. This being so, not only the quadrupole lens section QL2 and the cylindrical lens section CL but also the quadrupole lens section QL1 vary in lens power.
Owing to this, the divergence in the vertical direction may not be sufficient. In the case of a self-convergence type, the focusing force in the horizontal direction may abate though it must not. As a result, an electron beam spot on the periphery of the phosphor screen is excessively focused in the vertical direction, resulting in a halo portion, and is insufficiently focused in the horizontal direction.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 7-147146 proposes such an electron gun assembly as is shown in FIG. 5. The fifth grid of this electron gun assembly is made up of a first segment G51 and a second segment G52. As indicated by the broken lines in FIG. 6, a third grid and a second segment G52 are applied with a voltage that rises in accordance with the amount of deflection of an electron beam. As indicated by the broken lines in FIG. 7, therefore, a quadrupole lens QL3 having a diverging vertical component and a focusing horizontal component is formed between the first segment G51 and the second segment G52 in the deflection mode only.
If the auxiliary quadrupole lens section QL3 is formed, however, the lens main plane, namely, the imaginary lens center used for focusing electron beams on the phosphor screen (i.e., the point where the path of a beam emitted from the cathode and the path of a beam incident on the phosphor screen cross each other) is shifted.
In the non-deflection mode, the lens main plane in the vertical direction is located substantially in the center of the main lens section ML. In the deflection mode wherein the quadrupole lens section QL3 is used, the lens main plane in the vertical direction is shifted from the main lens section ML to the phosphor screen, i.e., in the increasing direction of the Z axis, since an electron beam is diverged in the vertical direction by the vertical-direction component of the quadrupole lens section QL3.
In the non-deflection mode, the lens main plane in the horizontal direction is located substantially in the center of the main lens section ML, just like the lens main plane in the vertical direction. In the deflection mode wherein the quadrupole lens section QL3 is used, the lens main plane in the horizontal direction is shifted from the main lens section ML to the cathode, i.e., in the decreasing direction of the Z axis, since an electron beam is focused by the horizontal-direction component of the quadrupole lens section QL3.
Owing to this movement of the lens main plane, in the phosphor screen periphery on which a deflected electron beam is focused, the angular magnification as viewed in the vertical direction is smaller than the angular magnification as viewed in the horizontal direction. As a result, the beam spot formed by the electron beam is not only influenced by the deflecting magnetic field generated by the deflecting yoke but also distorted in such a manner that it is elongated more in the horizontal direction than in the vertical direction.
On the periphery of the phosphor screen, the horizontal-direction diameter of the beam spot is very large, resulting in degradation in image quality. In addition, the vertical-direction diameter of the beam spot is very small, causing moire on the periphery.
In the case of a color cathode ray tube whose deflection angle is wide, the deflecting magnetic field inevitably includes a coma aberration component, and those components of the deflecting magnetic field which have effects on the lens function vary. In other words, the focusing effects which the deflection yoke lens may have on the side beams vary. As shown in FIG. 14, therefore, the beam spot diameters markedly differ between the right and left portions of the screen. Even if an adequate dynamic voltage is applied to the focusing electrodes, the electron beam spots on the right and left portions of the screen cannot be properly focused.
As described above, in the electron gun assembly disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-38947, the AC components of the dynamic focusing voltage applied to the fifth grid G5 are transmitted to the first and second intermediate electrodes through the capacitances between the electrodes of the main lens section ML. As a result, the lens power of the quadrupole lens section QL1 formed between the second intermediate electrode and the sixth grid varies. Since the diverging effect is insufficient in the vertical direction, and the focusing effect is insufficient in the horizontal direction, a beam spot formed on the periphery of the phosphor screen inevitably includes a halo portion which is due to the excessive focus in the vertical direction. In addition, the beam spot is elongated in the horizontal direction due to the insufficient focusing effect in this direction.
An electron gun assembly that has solved this phenomenal problem is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-147146. In the electron gun assembly described in this publication, an auxiliary quadrupole lens QL3 is formed on the cathode side of the main lens section ML in the deflection mode only. Owing to the operation of the quadrupole lens section QL3 in the deflection mode, the lens main plane as viewed in the vertical direction moves forward toward the screen, while the lens main plane as viewed in the horizontal direction moves backward toward the cathode. As a result, the lens power difference is caused between the vertical direction and the horizontal direction. Due to this difference, the beam spot is undesirably elongated in the horizontal direction.
In the case of a color cathode ray tube whose deflection angle is wide, the deflecting magnetic field inevitably includes a coma aberration component, and those components of the deflecting magnetic field which have effects on the lens function vary. In other words, the focusing effects which the deflection yoke lens may have on the side beams vary. As shown in FIG. 14, therefore, the beam spot diameters markedly differ between the right and left portions of the screen. Even if an adequate dynamic voltage is applied to the focusing electrodes, the electron beam spots on the right and left portions of the screen cannot be properly focused.
The present invention has been made to solve the above problems and is intended to provide a cathode ray tube which prevents or suppresses the distortion of a beam spot shape on the periphery of a screen and which therefore provides a reliable resolution at any portion of the screen.
As described above, the lens main plane as viewed in the horizontal direction moves backward toward the cathode, while the lens main plane as viewed in the vertical direction moves forward toward the screen. The angular magnification difference between the horizontal and vertical directions elongates an electron beam in the horizontal direction. This horizontal elongation, i.e., the angular magnification difference, becomes more marked in accordance with an increase in the intensity of the third quadrupole lens QL3. This phenomenon is attributed to the fact that the vertical and horizontal movements of the main lens plane are influenced by the focusing and diverging effects of the third quadrupole lens QL3. As described above, the lens operation of the third quadrupole lens QL3 is intended to compensates for the insufficient diverging effect of the vertical direction and the insufficient focusing effect of the horizontal direction, which are due to those AC components of the dynamic voltages which are superimposed on the voltages applied to the intermediate electrodes of the main lens section. As can be seen from this, if the superposing effects which the dynamic voltages may have on the intermediate electrodes can be suppressed, the lens effect of the third quadrupole lens QL3 need not be intense. In other words, the lens main plane does not move for a long distance in the horizontal and vertical directions, and the horizontal elongation which the electron beam spots may suffer on the periphery of the screen due to the angular magnification difference can be suppressed.
Hence, the elongation of an electron beam on the periphery of the screen can be suppressed by reducing the superposing effects which the dynamic voltages may have on the intermediate electrodes.
According to the present invention, the means for reducing the superposing effects which the dynamic voltages may have on the intermediate electrodes GM1 and GM2 is realized by the structure described below.
FIG. 9A shows the electrode structure and wiring pattern of the main lens section of the electron gun assembly that is applied to a cathode ray tube of the present invention. FIG. 9B shows an equivalent circuit of the main lens section shown in FIG. 9A.
A focusing electrode G52 is applied with an intermediate focusing voltage which may vary in synchronism with a deflecting magnetic field. A first anode G61 is applied with an anode voltage. Between these electrodes, at least one intermediate electrode GM is arranged, and the voltage applied thereto is higher than the intermediate focusing voltage and lower than the anode voltage. The three electrodes constitute a main lens section ML of an electric field expansion type. At least one auxiliary electrode G62 is arranged between the first anode G61 of the electric field expansion type main lens section ML and a second anode G63. This second anode G63 is located closer to the screen than the first anode 61, as viewed in the traveling direction of electron beams, and is applied with an anode voltage. The auxiliary electrode 62 and the intermediate electrode GM are electrically connected together.
In the descriptions given above, reference was made to the case where there is only one intermediate electrode GM, for the sake of simplicity. Needless to say, this in no way restricts the present invention, and a plurality of intermediate electrode may be provided. Although not illustrated in the electrode diagram, a third quadrupole lens QL3 is arranged on the cathode side of the focusing electrode G52.
In the case the electrode structure shown in FIG. 10A, the equivalent circuit is inevitably such as that shown in FIG. 10B according to the prior art. As can be seen from the equivalent circuit in FIG. 10B, the superposing voltage Vm applied to the intermediate electrode GM can be calculated by Vm=C/2Cxc2x7Vd=xc2xdxc2x7Vd, where the AC component of the dynamic voltage is Vd. Hence, 50% of the AC component Vd applied to the focusing electrode G52 is superimposed on the voltage applied to the intermediate electrode GM (on the condition that the capacitance between the focusing electrode G52 and the intermediate electrode GM is equal to that between the intermediate electrode GM and the anode G6). In contrast to this, the structure according to the present invention has an electrode structure such as that shown in FIG. 9A, and the equivalent circuit thereof is such as that shown in FIG. 9B. In this case, the superposing voltage Vm applied to the intermediate electrode GM can be calculated by Vm=C/4Cxc2x7Vd=xc2xcxc2x7Vd. Hence, 25% of the AC component Vd applied to the focusing electrode G52 is superimposed on the voltage applied to the intermediate electrode GM. In this manner, the structure of the present invention enables a reduction of superimposing voltage from 50% (prior art) to 25%.
In the prior art, 50% of the AC component of the dynamic voltage is superimposed on the voltage applied to the intermediate electrode GM of the main lens section, and this result in an insufficient diverging effect in the vertical direction and an insufficient focusing effect in the horizontal direction. To compensate for this insufficiency, the third quadrupole lens QL3 is used for moving the lens main plane in the horizontal direction backwards towards the cathode, and for moving the lens main plane in the vertical direction forwards towards the screen. Although an electron beam is horizontally elongated due to the angular magnification difference, this horizontal elongation can be reduced to half.
As shown in FIG. 11B, an asymmetric lens having a diverging effect in the vertical direction and a focusing effect in the horizontal direction is formed. It is formed between a first anode G61, a second anode G63, and an auxiliary electrode G62 for forming a main lens section of an electric field expansion type. The second anode G63 is located closer to the screen than the first anode G61, as viewed in the traveling direction of electron beams, and is applied with an anode voltage. The auxiliary electrode G62 is located between the first and second anodes G61 and G63 and is electrically connected to an intermediate electrode GM. The asymmetric lens is arranged in the neighborhood of the DY lens of a deflecting magnetic field.
FIG. 13A shows a lens state and an electron beam path of a case where an astigmatic lens is arranged in the neighborhood of the DY lens. FIG. 13B shows a lens state and an electron beam path of a case where an astigmatic lens is arranged at a position far away from the DY lens. In the Figures, xe2x80x9cxcex10xe2x80x9d represents radiating angles at the electron beam forming section, and xe2x80x9cxcex11(V)xe2x80x9d and xe2x80x9cxcex11(H)xe2x80x9d represent angles of incidence at which a beam is incident on the screen. xe2x80x9cLHxe2x80x9d and xe2x80x9cLVxe2x80x9d indicate the positions of the lens main planes in the vertical direction (V) and the horizontal direction (H). Let us assume that the beam radiating angle in the horizontal direction is equal to that in the vertical direction (=xcex10). When, in this case, the lens main plane positions are close to the cathode, the angles of incidence at which the beams fall on the screen are narrow, and the angular magnification is increased, accordingly. As a result, the electron beam spots projected on the screen are large. Conversely, when the lens main plane positions are close to the screen, the angular magnification is decreased, and the electron beam spots are small.
The case where the astigmatic lens is arranged in the neighborhood of the DY lens (FIG. 13A) and the case where it is arranged at a position far away from the DY lens (FIG. 13B) will be compared with each other. Where the astigmatic lens is located close to the DY lens, as shown in FIG. 13A, the lens main plane of the combination of the astigmatic lens and the DY lens is somewhat shifted from the DY lens towards the screen in the vertical direction (V), as indicated by xe2x80x9cLVxe2x80x9d, and is somewhat shifted from the astigmatic lens towards the cathode in the horizontal direction (H), as indicated by xe2x80x9cLH.xe2x80x9d Accordingly, the diameter of an electron beam is greater in the horizontal direction than in the vertical direction. This phenomenon becomes more marked when the astigmatic lens is arranged at a position far away from the DY lens, as in the case shown in FIG. 13B. In this case, the main plane position (LVxe2x80x2) as viewed in the vertical direction (V) is not shifted significantly. However, the main plane position (LHxe2x80x2) as viewed in the horizontal direction (H) is greatly shifted towards the cathode, further increasing the beam diameter of the electron beam spot in the horizontal direction. As can be seen from this, the astigmatic lens arranged in the neighborhood of the DY lens is more advantageous than the astigmatic lens arranged far away from the DY lens in that an electron beam spot shape can be as circular as possible even on the periphery of the screen.
As described above, the asymmetric lens serves not only to suppress those components of the dynamic voltage which are superimposed on the voltages applied to the intermediate electrode of the main lens section, but also to produce a diverging effect in the vertical direction and a focusing effect in the horizontal direction in the neighborhood of the DY lens. Owing to the formation of such an asymmetric lens, the electron beam spot on the periphery of the screen is prevented from being excessively distorted in the horizontal direction (an excessive decrease in the vertical diameter and an increase in the horizontal diameter are prevented).
To solve the problems and achieve the purpose, claim 1 provides a cathode ray tube comprising:
an electron gun assembly including: an electron beam formation section for forming and emitting at least one electron beam; and a main electron lens section for accelerating the electron beam and focusing the electron beam on a screen; and
a deflection yoke for generating a deflecting magnetic field, with which the electron beam emitted from the electron gun assembly is deflected to the screen in a horizontal direction and a vertical direction,
the main electron lens section being an electric field expansion type lens including: a focusing electrode applied with a focusing voltage of a first level; an anode applied with an anode voltage of a second level higher than the first level; and at least one intermediate electrode arranged between the focusing electrode and the anode and applied with an intermediate voltage of a third level which is higher than the first level and lower than the second level,
the anode including: a first anode; a second anode located closer to the screen than the first anode as viewed in the traveling direction of electron beams; and at least one auxiliary electrode interposed between the first anode and the second anode,
the at least one auxiliary electrode and the at least one intermediate electrode being electrically connected to each other.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.